This invention relates to a plaque for the rapid recognition of a dose of reactive and in particular of noxious gases by means of an indicator which undergoes a colour change on exposure to the gas. Such plaque may be used, for example, to obtain a retrospective reading of the dose of a noxious gas at a work-place. For this purpose, the plaque is attached to the work clothing of a person and carried during the whole working day. The indicator undergoes a characteristic colour change on exposure to a toxic gas. The intensity of the colour is normally proportional to lg.intg.c dt, where c is the concentration of the gas and t is the time. If a colour change occurs, the person carrying the plaque must immediately report to the doctor.
The colour change of the indicator has in the past been compared with a standard. Since such a method of assessment is time consuming and requires the use of apparatus, there is the risk that cases of poisoning may not be recognized sufficiently quickly. To enable the doctor to initiate optimum therapy at an early stage, he requires information on the possible dose inhaled.